The present invention generally relates to gas sensors and, more specifically, to an optoacoustic gas sensor.
Optoacoustic gas sensors have been used in various applications, including gas analyzers and gas detectors, to monitor environment gas concentration, most often for safety and process control purposes. Optoacoustic gas measurement is based on the same basic principles as conventional infrared-based gas analyzers—the ability of gases to absorb infrared light. More specifically, optoacoustic technology is based on generating acoustic pressure waves as result of gas irradiation with suitably modulated light.
A variety of optoacoustic gas sensors are known in the prior art. These include the optoacoustic gas sensors disclosed in U.S. Pat. No. 4,557,603 to Oehler et al.; U.S. Pat. No. 4,818,882 to Nexo et al.; U.S. Pat. No. 4,866,681 to Fertig; U.S. Pat. No. 5,753,797 to Forster et al.; U.S. Pat. No. 5,841,017 to Baraket et al. and U.S. Pat. No. 6,006,585 to Forster. These patents disclose arrangements where the sensor contains an infrared light source, a measurement cell, a membrane, a microphone and an electrical circuit for operating the light source and evaluating the microphone output signal. The light source transmits light pulses with specific wavelength into the measurement cell, where the radiation is absorbed by the gas to be detected and/or measured. Different gases can be measured by using light waves of different wavelengths corresponding to the absorption line of the gas to be measured. Therefore, the sensors may employ either narrow-band light sources, such as lasers, or wide-band light sources, such as incandescent filaments with optical band-pass filters.
When the gas to be detected is present in the environment within which the sensor is placed, the gas diffuses through the membrane into the measurement cell and there absorbs the light emitted. The absorbed radiation, which for very short time (a few microseconds) is stored as intermolecular vibrational-rotational energy, is quickly released by relaxation to translational energy during which numerous collisions occur between the gas molecules. Translational energy is equivalent to heat, which causes the pressure to rise in the absorption/measurement cell. If the incident light is modulated at a given frequency, a periodic pressure change is generated in the absorption cell.
At the edge of the measurement cell, the microphone captures the pressure modulation. The amplitude of the acoustic signal output is proportional to the radiation energy of the light source and the concentration of the absorbing gas. As a result, when the radiation energy is kept constant, the acoustic signal output can be measured and it delivers a value proportional to an absorbing concentration of gas.
There is quite frequently the need to detect and measure the concentration of multiple gases which are expected to be present or part of the process to be monitored. While it is possible to assign multiple sensors, each devoted to one specific gas, a single sensor distinctively measuring the concentration of multiple gases is undoubtedly a more economic alternative.
Furthermore, due to the nature of their detection principles, optoacoustic gas sensors have limited sensitivity. This is in part due to the effects of measurement cell wall effects, but is also due to environment pressure fluctuations and vibrations of several sources such as ventilation systems. It is therefore desirable to provide an optoacoustic gas sensor having a compensation cell in addition to the measurement cell that compensates for the interference signals generated exactly during gas measurement.
When it comes to reliability, the capability of gas sensors to have reliable self-diagnostic features to ensure the proper functionality of the sensor is undoubtedly most attractive. Most gas sensors do not have such capabilities. In addition, this capability must be technically feasible but also economically justifiable.
Gas sensors must be certified if they are to be installed in areas with intermittent or continuous presence of explosive gases (so-called “hazardous” or “classified” locations). To be certified, the design of a sensor must follow certain guidelines and fulfill strict requirements that are the subject of established standards. The most commonly applied standards for gas sensors relate to either explosion-proof or intrinsically safe design methods. It is therefore desirable to provide a sensor design that takes into consideration the most stringent certification requirements and fulfills either or both design methods.
Power consumption is becoming a crucial issue in the field of gas sensors as they are being increasingly used to detect and measure different gases from remote locations. Providing real-time measurements from distant locations puts a burden on sensors in terms of power consumption. It is therefore desirable to provide a gas sensor that features low power consumption.
It is also desirable to provide a gas sensor that corrects the effects of varying temperature so as to improve measurement accuracy.
It is also advantageous for the sensor output to represent not only the actual gas concentration, but also the gas concentration rate of increase, once a preset threshold has been reached or exceeded. For example, when a sensor is used to detect and measure natural gas (CH4) leaks, the rate of gas concentration increase may provide additional information regarding the severity of the leakage situation.
Accordingly, it is an object of the present invention to provide an optoacoustic gas sensor that can simultaneously measure the presence and concentration of multiple gases.
It is another object of the present invention to provide an optoacoustic gas sensor that compensates for interference signals.
It is another object of the present invention to provide an optoacoustic gas sensor that has reliable self-diagnostic features.
It is another object of the present invention to provide an optoacoustic gas sensor that may be certified as explosion-proof and/or intrinsically safe.
It is still another object of the present invention to provide an optoacoustic gas sensor that has low power consumption.
It is still another object of the present invention to provide an optoacoustic gas sensor that corrects for varying temperature.
These and other objects and advantages will be apparent from the following specification.